Hydrogen storage compositions and methods of manufacture thereof

ABSTRACT

Disclosed herein is a method for making and screening a combinatorial library comprising disposing in a substrate comprising boron, boron nitride, or boron carbide at least one reactant, wherein the reactants are lithium, magnesium, sodium, potassium, calcium, aluminum or a combination comprising at least one of the foregoing reactants; heat treating the substrate to create a diffusion multiple having at least two phases; contacting the diffusion multiple with hydrogen; detecting any absorption of hydrogen; and/or detecting any desorption of hydrogen.

BACKGROUND

This disclosure is related to hydrogen storage compositions and methodsof manufacture thereof.

Hydrogen is a “clean fuel” because it can be reacted with oxygen inhydrogen-consuming devices, such as a fuel cell or a combustion engine,to produce energy and water. Virtually no other reaction byproducts areproduced in the exhaust. As a result, the use of hydrogen as a fueleffectively solves many environmental problems associated with the useof petroleum based fuels. Safe and efficient storage of hydrogen gas is,therefore, essential for many applications that can use hydrogen. Inparticular, minimizing volume and weight of the hydrogen storage systemsare important factors in mobile applications.

Several methods of storing hydrogen are currently used but these areeither inadequate or impractical for wide-spread mobile consumerapplications. For example, hydrogen can be stored in liquid form at verylow temperatures. However, the energy consumed in liquefying hydrogengas is about 40% of the energy available from the resulting hydrogen. Inaddition, a standard tank filled with liquid hydrogen will become emptyin about a week through evaporation; thus dormancy is also a problem.These factors make liquid hydrogen impractical for most consumerapplications.

An alternative is to store hydrogen under high pressure in cylinders.However, a 100 pound steel cylinder can only store about one pound ofhydrogen at about 2200 psi, which translates into 1% by weight ofhydrogen storage. More expensive composite cylinders with specialcompressors can store hydrogen at higher pressures of about 4,500 psi toachieve a more favorable storage ratio of about 4% by weight. Althougheven higher pressures are possible, safety factors and the high amountof energy consumed in achieving such high pressures have compelled asearch for alternative hydrogen storage technologies that are both safeand efficient.

In view of the above, there is a need for safer, more effective methodsof storing and recovering hydrogen. In addition, there is a desire tominimize the overall system volume and weight.

BRIEF DESCRIPTION OF THE INVENTION

Disclosed herein is a method for making and screening a combinatoriallibrary comprising disposing in a substrate comprising boron, boronnitride, or boron carbide at least one reactant, wherein the reactantsare lithium, magnesium, sodium, potassium, calcium, aluminum or acombination comprising at least one of the foregoing reactants; heattreating the substrate to create a diffusion multiple having at leasttwo phases; contacting the diffusion multiple with hydrogen; detectingany absorption of hydrogen; and/or detecting any desorption of hydrogen.

Disclosed herein is a method of recovering hydrogen comprisingcontacting at least one compound selected from the group consisting ofAlB₂, AlB₁₂, B₆Ca, B₆K, B₁₂Li, B₆Li, B₄Li, B₃Li, B₂Li, BLi, B₆Li₇, BLi₃,MgB₂, MgB₄, MgB₇, NaB₆, NaB₁₅, NaB₁₆ or a combination comprising atleast one of the foregoing borides with hydrogen to form a hydrogenatedcompound; and heating the hydrogenated compound to recover the hydrogen.

Disclosed herein too is a method of recovering hydrogen comprisingcontacting at least one compound of a diffusion multiple, wherein thediffusion multiple has the formula (II)((Li_(a), Na_(b), K_(c), Al_(d), Mg_(e), Ca_(f))_(x)(B, C, N)_(y)  (II)where Li is lithium, Na is sodium, Mg is magnesium, K is potassium, Cais calcium, Al is aluminum; B is boron, C is carbon and N is nitrogen;a, b, c, d, e and f may be the same or different and have values from 0to 1; and x and y have values of about 1 to about 22; in hydrogen toform a hydrogenated compound; and heating the hydrogenated compound torecover the hydrogen.

Disclosed herein too is a diffusion multiple comprising compounds havingthe formula (II)((Li_(a), Na_(b), K_(c), Al_(d), Mg_(e), Ca_(f))_(x)(B, C, N)_(y)  (II)where Li is lithium, Na is sodium, Mg is magnesium, K is potassium, Cais calcium, Al is aluminum; B is boron, C is carbon and N is nitrogen;a, b, c, d, e and f may be the same or different and have values from 0to 1; and x and y have values of about 1 to about 22.

Disclosed herein too is a composition comprising a hydride of acompound, wherein the compound is AlB₂, AlB₁₂, B₆Ca, B₆K, B₁₂Li, B₆Li,B₄Li, B₃Li, B₂Li, BLi, B₆Li₇, BLi₃, MgB₂, MgB₄, MgB₇, NaB₆, NaB₁₅, NaB₁₆or a combination comprising at least one of compounds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing the arrangement of a diffusion multipleassembly comprising a boron substrate;

FIG. 2 is a schematic showing how the diffusion multiple assembly issliced for purposes of analysis; and

FIG. 3 is a schematic showing a system for the generation of hydrogenfrom the hydrides of light metal silicides, borosilicides,carbosilicides and nitrosilicides.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Disclosed herein is a method for developing a combinatorial library todetermine borides, borocarbides and boronitrides that may beadvantageously used for the storage of hydrogen. Disclosed herein tooare methods for manufacturing borides, borocarbides and boronitridesthat can be subsequently hydrogenated to efficiently store hydrogen.Disclosed herein too are compositions comprising borides, borocarbidesand boronitrides that can store hydrogen for use in the generation ofenergy in fuel cell applications for automobiles, homes and apartments,manufacturing industries, and the like. The aforementioned method fordeveloping a combinatorial library to determine borides, borocarbidesand boronitrides that may be used for the storage of hydrogenadvantageously permits the simultaneous large scale testing of a widevariety of materials. This high efficiency methodology facilitates thecreation of a large number of controlled compositional variations inbulk samples for fast and systematic surveys of hydrogen storageproperties of the borides, borocarbides and boronitrides.

Complex hydrides from which hydrogen can be obtained generally consistof a H-M complex, where M is a metal and H is hydrogen. Such hydridesmay have ionic, covalent, metallic bonding or bonding comprising acombination of at least one of the foregoing types of bonding. Thesehydrides preferably have a hydrogen to metal ratio of greater than orequal to about 1. The reaction between a metal and hydrogen to form ahydride is generally a reversible reaction and takes place according tothe following equation (I):M+(x/2) H₂

MHx  (I)Complex hydrides can store up to about 18 weight percent (wt %) ofhydrogen, and have high volumetric storage densities. The volumetricstorage density of hydrides is greater than either liquid or solidhydrogen, which makes them very useful in energy storage applications.The process of hydrogen adsorption, absorption or chemisorption resultsin hydrogen storage and is hereinafter referred to as absorption, whilethe process of desorption results in the release of hydrogen.

In an exemplary embodiment, compositions comprising light metal borides,borocarbides and boronitrides can form hydrides that may be reversiblydecomposed at relatively low temperatures of less than or equal to about300° C. to release hydrogen. The light metals are alkali metals and/oralkaline earth metals. Preferred light metals are lithium, sodium,magnesium, potassium, aluminum and calcium. The borides, borocarbidesand boronitrides have the formula (II)(Li_(a), Na_(b), Mg_(c), K_(d), Ca_(e), Al_(f))_(x)(B, C, N)_(y)  (II)where Li is lithium, Na is sodium, Mg is magnesium, K is potassium, Cais calcium, Al is aluminum; B is boron, C is carbon and N is nitrogen;a, b, c, d, e and f may be the same or different and have values from 0to 1; and x , and y have values of about 1 to about 22. The sum ofa+b+c+d+e+f is preferably equal to 1.

In one embodiment, a method of developing a combinatorial library fordetermining the hydrogen storage capabilities of a borides, borocarbidesand boronitrides is via the use of a diffusion multiple. A diffusionmultiple is a compound that is the product of an interdiffusion reactionformed between a first reactant and a second reactant, when bothreactants are placed in closed proximity with each other and heated to atemperature effective to permit interdiffusion to take place. Thereactants may be elements or if desired may be compounds or alloys. Thetemperature effective to permit the interdiffusion is one that canovercome the activation energy of diffusion and achieve at least adegree of interdiffusion of the reactants within a manageable time. Sucha temperature is generally about 200 to about 2000° C., depending uponthe reactants.

Diffusion multiples are generally manufactured or prepared by placingreactants in a substrate to form a diffusion multiple assembly;optionally subjecting the diffusion multiple assembly to hot isostaticpressing; heat treating the diffusion multiple assembly to promoteinterdiffusion of the reactants with one another and/or interdiffusionbetween the reactants with the substrate; optionally cutting, polishingand grinding the diffusion multiple; identifying the elementalcomposition of the various phases present in the diffusion multiple; andcharging the diffusion multiple with hydrogen by contacting with ahydrogen rich gaseous mixture and determining the phases that absorbhydrogen.

In one exemplary method of making a diffusion multiple assemblycomprising light metal borides, borocarbides and boronitrides, thediffusion multiple is prepared by drilling holes into a boron, a boronnitride (BN), or a borocarbide (B₄C) substrate. These holes generallyend half-way through the thickness of the substrate. Some holes arespaced apart from one another such that during the heat treatment, thereis only one reactant reacting with the substrate material to form binarycouples and binary solid solutions. When the substrate comprises boronnitride or a borocarbide, ternary triples may be formed. In anothermanner of making a ternary triple, holes are spaced in close proximityin pairs with each other as shown in the FIG. 1. This arrangement, i.e.,where the holes are spaced in close proximity in pairs may be used togenerate ternary diffusion triples (also termed ternary compounds and/orternary solid solutions) upon subjecting the diffusion multiple assemblyto heat treatment. The reactants are generally placed into the holes ina loose form i.e., they do not need to be a tight fit.

The number of holes drilled in the substrate is generally equal to theminimum number of diffusion multiples desired. Thus for example, if abinary diffusion couple is desired in a substrate made from a singleelement such as silicon, one hole is drilled into the substrate, whileif a ternary diffusion triple is desired, two holes are drilled into thesubstrate in close proximity to one another. As stated above, anothermethod of making a ternary triple comprises drilling a single hole intoa substrate, wherein the substrate is made up of an alloy. The holes areabout 1 to about 10 millimeters in diameter. The preferred diameter isabout 5 millimeter. The thickness of the substrate is generally about 5to about 25 millimeters in diameter. The preferred thickness of thesubstrate is about 25 millimeters.

The distance between the holes in the substrate is maintained as closeas possible for those drilled in pairs. The distance d is generallyabout 0.1 to about 2000 micrometers. Within this range, it is generallydesirable to utilize the distance to be less than or equal to about 400,preferably less than or equal to about 200, and more preferably lessthan or equal to about 100 micrometers.

In an exemplary embodiment, in one manner of proceeding, a diffusionmultiple assembly comprises a boron substrate as shown in FIG. 1. Theboron substrate is used to prepare a combinatorial library from alkalimetals and/or alkaline earth metals. In other words, the alkali metalsand/or the alkaline earth metals are placed in the holes in thesubstrate to form the diffusion multiples. The substrate has a diameterof 2.0 inches and the holes containing the reactants are drilled to adepth of 0.5 inch. The reactants selected for placement in the holes inthe substrate are potassium, lithium, sodium, magnesium, aluminum andcalcium. As may be seen from the FIG. 1, the reactants magnesium,sodium, potassium, lithium and aluminum are placed into individual holesin the substrate. These may be used to prepare binary diffusion couplesof the reactants with boron.

Ternary diffusion triples of the reactants may also prepared by drillingholes in close proximity to each other as may be seen in FIG. 1. Theternary triples comprise magnesium and aluminum with boron, sodium andaluminum with boron, magnesium and potassium with boron, lithium andaluminum with boron, sodium and magnesium with boron, sodium andpotassium with boron, lithium and sodium with boron, lithium andmagnesium with boron, lithium and potassium with boron, and sodium andaluminum with boron.

The operation of placing the light metals into the hole in the substrateis carried out in a well-controlled environment such as a glove boxfilled with pure argon to prevent the light-elements from oxidation. Theamount of light-elements in each hole is usually less than a quarter ofthe volume of the hole such that there will be no pure light elementsleft after the interdiffusion/heat treatment step. The boron substratewith the light-elements in the holes are then transferred to a furnaceor a reactor. The furnace or reactor is either in a vacuum or aprotective environment such as argon. The substrate is then heated to anelevated temperature to allow significant interdiffusion to take placeamong the elements in the holes and the boron substrate.

The substrate is preferably heat treated from a temperature of about 660to about 1000° C. to permit the melting of the reactants or theireutectic compositions. The heat treatment is generally conducted in aconvection furnace. The heat treatment to form the diffusion multiplemay also include using radiant heating and/or conductive heating ifdesired. The melted reactants diffuse and react with the boron substrateto form borides, doped phases, and solid-solution compositions. Whenboron carbide is used as the substrate, the heat treatment is generallyconducted at temperatures of about 660 to about 1,250° C. so that theformation of a diffusion multiple is facilitated within a reasonabletime. The preferred temperature for heat treatment is 700° C. When boronnitride is used as the substrate, the heat treatment is generallyconducted at temperatures of about 660 to about 1,250° C. so that theformation of a diffusion multiple is facilitated within a reasonabletime. The preferred temperature for heat treatment is 700° C.

In one embodiment, in one method of manufacturing a diffusion multiplecomprising a borocarbide, a boron substrate with the desired reactantsis subjected to heat treatment in a carbonaceous atmosphere. Thediffusion multiples are generally ternary triples comprising aborocarbide. The heat treatment temperature for the preparation of theborocarbides is about 660 to about 2000° C.

The time period of the heat treatment of the diffusion multiple assemblyis about 5 to about 100 hours. Within this range, it is generallydesirable to heat treat the diffusion multiple assembly for greater thanor equal to about 10, preferably greater than or equal to about 20, andmore preferably greater than or equal to about 25 hours. Also desirablewithin this range is a time period of less than or equal to about 75,preferably less than or equal to about 50 and more preferably less thanor equal to about 40 hours. An exemplary time period of heat treatmentat a temperature of 700° C. is about 24 hours.

After the heat treatment to form the diffusion multiple, a slicingoperation may be performed on the diffusion multiple assembly. Theslicing step is designed to expose different compounds/solid solutionsformed at different locations of the diffusion multiple assembly asshown in FIG. 2. The slicing operation is generally performed usingmechanical cutting using a saw or wire discharge electro-machining(EDM). Following slicing, the respective slices may be optionallysubjected to grinding and polishing if desired. Following the optionalgrinding and polishing operation, the samples are subjected to electronmicroprobe analysis and electron backscatter diffraction (EBSD) analysisto identify (i.e., locate) and analyze the phases and compounds prior tobeing tested for the ability of the light metal borides, borocarbidesand boronitrides for hydrogenation.

After the electron microprobe and EBSD analysis of the light metalborides, borocarbides and boronitrides, the resulting compositions inthe diffusion multiples may be converted to hydrides by exposure tohydrogen or upon hydrogenation.

The following borides can be obtained from the diffusion multipleassembly shown in FIG. 1 and may be used for a determination of hydrogenpotential: AlB₂, AlB₁₂, B₆Ca, B₆K, B₁₂Li, B₆Li, B₄Li, B₃Li, B₂, BLi,B₆Li₇, BLi₃, MgB₂, MgB₄, MgB₇, NaB₆, NaB₁₅, NaB₁₆, or a combinationcomprising at least one of the foregoing borides or the like. Ternaryborides in general, and ternary borides comprising the foregoing boridesin particular may be useful for hydrogenation and for generatinghydrogen. In addition borocarbides and boronitrides of lithium,magnesium calcium, sodium, potassium and aluminum may also be used forhydrogenation and for generating hydrogen.

The borides, borocarbides and boronitrides generally have at least oneof potassium, lithium magnesium, calcium or sodium. The presence of thepotassium, lithium, magnesium, calcium and sodium promotes an affinityfor hydrogen. Boron on the other hand has a low affinity for thehydrogen and this feature is offset by the affinity of hydrogendisplayed by calcium, potassium, lithium, magnesium and/or sodium.Without being limited to theory it is believed that those elements ofthe diffusion multiple that have a high affinity for hydrogen generallyfacilitate absorption of hydrogen, while those elements such as boronthat have a low affinity for hydrogen generally facilitate thedesorption.

The diffusion multiple comprising the light metal borides, borocarbidesand boronitrides can generally be tested for their ability to absorb anddesorb hydrogen. The composition gradients formed during the preparationof a diffusion multiple can serve as a combinatorial library todetermine which specific composition can absorb and desorb hydrogen.

The ability of a light metal boride, borocarbide or boronitride in thediffusion multiples to reversibly absorb and desorb hydrogen may bedetected by a variety of analytical techniques. In general, the processof absorption of hydrogen into the borides, borocarbides andboronitrides results in a change in appearance because of a crystalstructure change and/or a volumetric expansion. In addition, theabsorption of hydrogen into the borides, borocarbides and boronitridesis generally accompanied by an exotherm, while the desorption of thehydrogen from the borides, borocarbides and boronitrides is generallyaccomplished by the application of heat. The analytical techniques thatcan be used to measure the changes in the diffusion multiples are timeof flight secondary mass ion spectrometry (ToF-SIMS), tungsten oxide(WO₃) coatings and thermography. In addition, the borides, borocarbidesand boronitrides can be screened by observing the diffusion multipleafter hydrogenation, since the phases that do undergo hydrogenation(i.e., hydrides) generally become pulverized.

The ToF-SIMS has the capability to detect the absorption and desorptionof all elements including hydrogen, which makes it useful for thedetermining those compositions present in the light metal diffusionmultiple that can readily be used for the storage of hydrogen. Thistechnique can operate at temperatures of about −100 to about 600° C.,has a high sensitivity to hydrogen and is therefore a useful tool forinvestigating the combinatorial libraries generated by the diffusionmultiples. The ToF-SIMS can therefore be effectively used to map theabsorption temperatures and the reaction conditions during thehydrogenation process.

The tungsten oxide (WO₃) generally changes its color when it reacts withhydrogen. In order to use the tungsten oxide as a detector for thehydrogen uptake in the various compositions of the diffusion multiple,the diffusion multiple is coated with WO₃ after the hydrogenationreaction. When the diffusion multiple is heated up to release thehydrogen, the WO₃ changes color as the hydrogen desorbs from thediffusion multiple.

Thermography or thermal imaging (infrared imaging) may also be used todetermine the absorption and desorption of hydrogen. When a phase in thediffusion multiple absorbs hydrogen, the local temperature rises, whilewhen the phase desorbs hydrogen, the local temperature decreases.Thermography can therefore be used to image the compounds that absorb ordesorb hydrogen.

In one embodiment, the borides, borocarbides and boronitrides can behydrogenated by subjecting them to a mixture of gases comprisinghydrogen. As stated above, the borides, borocarbides and boronitridesgenerally release heat during the absorption of hydrogen. The hydrogenmay then be released by reducing the pressure and supplying heat to thehydrogenated borides, borocarbides and boronitrides. The desorption ofhydrogen often requires thermal cycles. Such thermal cycles can beobtained by the application of electromagnetic fields or by passingelectrical current through the material of interest. This can beaccomplished because most hydrogenated borides, borocarbides andboronitrides are electrically conductive. The resistance of thesematerials changes with the extent of hydrogen storage.

In one embodiment, the desorption of stored hydrogen can be facilitatedby the use of electromagnetic fields. Microwave energy can be directlyapplied to the hydrogenated boride, borocarbide or boronitride or to asuitable medium such as water, alcohols, or the like, intermixed withthe hydrogenated borides, borocarbides and boronitrides to allow for thelocal release of hydrogen under controlled conditions, without heatingthe whole system. This method provides a high efficiency of desorption,which generally occurs at temperatures lower than those achieved due toheating brought about by conduction and/or convection. This phenomenaoccurs due to a local excitation of the bonds in the boride, borocarbideor boronitride by the microwaves. The desorption may be conducted by twodifferent methods. The first of these methods comprises using microwavesto achieve a release of the entire hydrogen content. The second methodcomprises using a microwave treatment just to initialize the desorptionprocess which then can be continued by either conductive and/orconvective heating at lower temperatures and in a much easier mannerthan when heated by only conductive and/or convective heat from thestart of the process.

In yet another embodiment, hydrogen desorption can be induced by theheat generated by an electrical resistor embedded in the borides,borocarbides and boronitrides. The energy of the current flowing intothe resistor is converted into heat by the Joule effect. The amount ofheat created locally by the current flow is particularly high in thecase of a compressed powdered boride material, with hot spots occurringon the current paths between powder particles, where the resistivity isvery high. In extreme cases, powder welding may occur at the hot spots.Therefore, the current parameters should be adjusted properly to avoidsintering or powder welding. Depending on the conditions of the process,the borides, borocarbides and boronitrides may be heated directly, or bythe use of multiple resistors as detailed above.

In yet another embodiment, hydrogen absorption and desorption isaccomplished by mixing fine particles of the borides, borocarbides andboronitrides with an appropriate amount of another chemical compositionthat has a higher thermal conductivity to conduct heat faster to thehydrogenated compound for hydrogen release. In yet another embodiment,hydrogen desorption is accomplished by using the exhaust heat releasedfrom the proton exchange membrane (PEM) fuel cells to heat up thehydrogenated borides, borocarbides and boronitrides.

In yet another embodiment, dopants comprising titanium, vanadiumzirconium, yttrium, lanthanum, nickel, manganese, cobalt, boron,gallium, germanium, and the elements from the lanthanide series may beadded to catalyze the desorption of hydrogen. The dopant may be added inan amount of up to about 20 wt %, of the total hydrogen storagecomposition prior to the storage of hydrogen. It is generally desirableto add the dopant in an amount of less than or equal to about 15,preferably less than or equal to about 10 and more preferably less thanor equal to about 5 wt % of the total weight of the hydrogen storagecomposition (i.e., the diffusion multiple) prior to the storage ofhydrogen.

The hydrogen desorbed from these borides, borocarbides and boronitridescan be about 1 to about 8 wt %, with amounts of greater than or equal toabout 4 wt % preferred, amounts of greater than or equal to about 5 wt %more preferred, and amounts of greater than or equal to about 6 wt %even more preferred.

As stated above, the combinatorial method of determining the capabilityof light metal borides, borocarbides and boronitrides to absorb anddesorb hydrogen is quick and efficient. The light metal borides,borocarbides and boronitrides that are determined to absorb and desorbhydrogen may be utilized in fuel cells, gas turbines, and the like forthe storage of energy.

In one exemplary method of producing and storing hydrogen from hydridesof the light metal borides, borocarbides and boronitrides, a systemshown in FIG. 3 comprises an optional slurry production reactor inupstream of and in fluid communication with a hydrogen generationreactor. The slurry production reactor regenerates a metal hydrideslurry that is utilized to produce hydrogen in the hydrogen generationreactor. At least a portion of the metal hydride in the hydrogengeneration reactor is oxidized to a metal hydroxide during the recoveryof hydrogen from the light metal hydrides. The hydrogen generationreactor utilizes electromagnetic radiation, convectional heating, PEMfuel cell exhaust, and the like to heat the hydride for the generationof hydrogen. The hydrogen generation reactor is also upstream of and influid communication with an optional drying and separation reactor andthe metal hydroxide is transferred to the drying and separation reactor.At least a portion of metal hydroxide generated in the hydrogengeneration reactor is recycled to the drying and separation unit. Thehydrogen generation reactor is optionally supplied with water. Theoptional drying and separation reactor separates any reusable fluidssuch as water from the metal hydroxides and recycles the fluid to theoptional slurry production reactor. The system also comprises a hydriderecycle reactor in fluid communication with and downstream of the dryingand separation unit. Dry metal hydroxide from the drying and separationreactor is regenerated into a metal hydride in the hydride recyclereactor by contacting it with a mixture of gases comprising hydrogen.The hydride recycle reactor is supplied with carbon and oxygen inamounts effective to regenerate the metal hydride. The regenerated metalhydride is then recycled to the slurry production reactor for mixingwith the recycled carrier liquids.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A method of recovering hydrogen comprising: contacting at least onecompound selected from the group consisting of AlB₂, AlB₁₂, B₆Ca, or acombination comprising at least one of the foregoing borides withhydrogen to form a hydrogenated compound; and heating the hydrogenatedcompound to recover the hydrogen.
 2. The method of claim 1, wherein theheating is conducted using microwave radiation, convectional heating, ora combination comprising both of the foregoing methods of heating. 3.The method of claim 1, further adding a dopant comprising titanium,vanadium zirconium, yttrium, lanthanum, nickel, manganese, cobalt,boron, gallium, germanium, and the elements from the lanthanide seriesto the compound in an amount of less than or equal to about 20 wt % ofthe diffusion multiple.
 4. The method of claim 1, wherein the heating iseffected by the heat from the exhaust of a fuel cell.